What have I done that causes the one face to be effected differently to the subsurf modifier?
The face was in fact detached. At some point I must have used the Hotkey Y and split the face whilst the face was selected. According to the blender Manual this disconnects (splits) the selection from the rest of the mesh and the border edge to any non-selected elements are duplicated and the “copy” is left exactly at the same position as the original.
To fix this I used the Remove Doubles tool which according to the Blender Manual merges the selected vertices that are closer than a specified distance to each other. 

In Vertex Mode, Select all, Ctrl-V (shortcut to Vertex menu), Remove Doubles.

This answer was in fact provided by Robin Betts in the comments section. Many thanks to him.